Device for plasma arc treatment of materials



United States Patent Inventors Appl. No. Filed Patented Priority DavidGrigorievich Bykhovsky, Konjushenny pereulok, [/6 kv. 18; AlexandrYakovlevich Medvedev, Ulitsa Kalyaeva, 3, kv .17; Yakov Vull'ovichRossomakho, Ulitsa Treloleva, 32, kv. l4; Valery Alexandrovich Koss,Ulitsa Besstuzhevskaya 11, kv. 56, all of Leningrad, U.S.S-.R.

Feb. 11, 1969 Dec. 8, 1970 Feb. 15, 1968 U.S.S.R.

DEVICE FOR PLASMA ARC TREATMENT OF MATERIALS 4 Claims, 1 Drawing Fig.

U-S-Cl. 219/121, 219/75 lnt.C1. 3231:9/26

Field ofSearch 219/74, 75,

\ \IQ K Primary Examiner.l. V. Truhe Attorney-Holman, Giascock, Downing& Seebold ABSTRACT: A device for plasma-arc machining of materials inactive media, preferably for cutting metals, wherein the cathode holderand cathode are separated by a gasket which increases the cathode lifeup to 6 hours at an allowable arc current of up to 350a. The gasketmaterial is inert with respect to the cathode material (zirconium) andthe cathode holder material (copper or copper alloy) and is preferablyselected from the following: tungsten, tantalum, molybdenum, niobium,vanadium or chromium.

PATENTED DEC 8 I970 *WIL DEVICE FOR PLASMA ARC TREATMENT OF MATERIALSThe present invention relates to devices for plasma-arc treatment ofmaterials and more particularly it relates to the devices for plasma-arctreatment of materials in active media, preferably for cutting metals.

The device can also be employed for surface fusion, for example for theremoval of a defective surface layer from castings and cast products orfor the correction of such local defects as cracks, pits, scabs. Theinvention can also be used for heating chemically active gases and theirmixtures and for the thermochemical surface treatment of metals, e.g.nitriding, case-hardening and similar processes.

The present invention'can be used both with and without additionalprotection of the cathode assembly.

Known in the art is a device for plasma-arc treatment of materials inactive media, preferably for cutting metals, comprising a cathode holdermade of copper or a copper base alloy, and accommodating a zirconiumcathode. The cathode holder is connected, via an insulting bushing, withthe body accommodating the nozzles which stabilize the arc discharge.

A substantial disadvantage of the known device is an extremely shortservice life of the zirconium cathode (not over 4 hour) at are currentsof 250.

One of the main causes of excessive wear of the zirconium cathode isdeterioration of the thermal and electrical contact between thezirconium cathode and the copper cathode holder. The temperature on thecontacting surface between zirconium and copper reaches 700 1 ,000 C.The diffusion processes at such temperatures produce intermetalliccompounds at thepoint of contact between copper and zirconium, thesecompounds having a-farlower heat and electrical conductivity than copperor zirconium. The layer of intermetallic compoundinterferes with thewithdrawing of heat from zirconium which leads eventually to itsultimate destruction.

An object of the present invention is to eliminate the aforesaiddisadvantage.

The main object of the present invention is to provide a device. forplasma-arc treatment of materials in active media, preferably forcutting metals, whose cathode would be more wear resisting as comparedto the known device at arc currents of250 a. and above. i

This object is accomplished by providing a device for plasma-arctreatment of materials in active media, preferably for cutting metals,comprising a cathode holder made of copper or a copper base alloy with azirconium cathode.

According to the invention, the cathode holder is separated from thecathode by a gasket made of a material which is chemically inert withrelation to both zirconium and copper.

Now the invention will be described in detail by way of example withreference to the drawing (which shows schematically a device forplasma-arc cutting of metals in active media).

The device for plasma-arc cutting of metals in active media consists ofa body 1 accommodating a cathode holder 2 and a nozzle 3 with a centralpassage 4 intended to stabilize the arc discharge 5.

To prevent formation of a double arc in case the nozzle 3 accidentallycomes in contact with the work 6, or to increase the total length of thestabilizing central passage 4, there may be installed at least one morenozzle 7 (shown by dotted lines in the drawing).

The cathode holder 2 made of copper or a copper base alloy, is fastenedin the body 1 by means of the insulating bushing 8, and is cooled by thecoolant supplied through a pipe 9 installed inside the cathode holder.The nozzles 3 and 7 have passages 10 and 11 for the supply of coolant.

lf a number of nozzles are used, they are separated by an insulatinggasket 12. The coolant may be either water or gas. The body has a hole13 for the supply of plasma-generating gas, e.g. air, nitrogen, oxygen,or mixtures containing nitrogen and oxy en.

Pressitted into the cathode holder 2 (flush with it) is the zirconiumcathode 14 separated from the copper cathode holder 2 by the gasket 15which is made of a metal which is inert both to zirconium and copper(tungsten, tantalum, molybdenum, niobium, vanadium, chromium).

The thickness of this gasket should be sufficient to rule out thediffusion of copper into zirconium. For instance, at temperatures up tol,000 C. the thickness of the gasket may be l0- mm.

The zirconium cathode 14 can be separated from the copper cathode holder2 by the gasket 15 using one of the following methods: byelectrodeposition of one of the abovementioned metals or their compoundson the cathode 14, or by vacuum spraying of these metals on the cathode14 or cathode holder 2. The separating gasket 15 can also be fitted intothe cathode holder by a mechanical method.

The cathode holder 2 is acted upon by a negative potential while thework is under the positive potential. The are discharge 5 burns betweenthe zirconium cathode l4 and the work 6, passing through the stabilizingcentral passage 4 in the nozzle 3. If the nozzle 7 is used, the arcdischarge takes place between the cathode 14 and the work 6, passingthrough the stabilizing central passages 4 and 16 of the nozzles 3 and7, respectively.

The tests of the claimed device have revealed that the allowable arccurrent has been increased to 350 a. with a simultaneous extension ofthe life of the zirconium cathode to 6 hour.

We claim:

1. In a plasma generator having a central cathode holder and cathode, anannular cavity between the cathode holder and a housing, plasmagenerating gas inlet means to the cavity, at least one central nozzlebetween the cathode and a workpiece, and a power source, the improvementconsisting in that a conductive metallic gasket is interposed betweensaid cathode holder and cathode, said cathode being made of zirconium,said cathode holder being made of a metal selected from the groupconsisting essentially of copper or copper alloy, and said metallicgasket being made of a metal which is inert to either said cathode orholder with regards to forming intermetallic compounds therewith atoperating temperature.

2. A plasma generator as claimed in claim 1 wherein said cathode holderand central nozzle have coolant passages therein.

3. A plasma generator as claimed in claim 1 wherein said gasket is madeof a metal selected from the group consisting essentially of tungsten,tantalum, molybdenum, niobium, vanadium or chromium.

4. A plasma generator as claimed in claim 3 wherein said operatingtemperature is approximately 1,000 C. and said gasket is approximately10- mm. thick.

